U.S. patent number 6,885,125 [Application Number 10/931,421] was granted by the patent office on 2005-04-26 for brushless dc motor and method of manufacturing brushless dc motor.
This patent grant is currently assigned to Koyo Seiko Co., Ltd.. Invention is credited to Tetsuo Horie, Hirohide Inayama, Atsushi Ishihara, Hideki Jonokuchi, Minoru Kitabayashi, Sadaaki Mori, Tomofumi Takahashi.
United States Patent |
6,885,125 |
Inayama , et al. |
April 26, 2005 |
Brushless DC motor and method of manufacturing brushless DC
motor
Abstract
A brushless DC motor including a stator having plural slots; and
a rotor which has plural permanent magnets and is divided into
three rotor blocks in a rotation axis direction, the three rotor
blocks being layered so that the arrangement angles of the rotor
blocks differ from each other by an amount of a mechanical angle in
a rotary direction that is equivalent to one third of a pulsation
period of cogging torque generated by the rotor and stator. A
brushless DC motor including a rotor having plural magnetic poles
provided at an equal pitch in a circumferential direction by
mounting permanent magnets in magnet mounting holes; and a stator
having plural slots arranged at an equal pitch in a circumferential
direction. The magnetic poles of the rotor include magnetic poles
whose magnet deviation angle formed by the central line of an
effective polar opening angle and the central line of the magnet
mounting hole is the first angle; and magnetic poles whose magnet
deviation angle is the second angle different from the first
angle.
Inventors: |
Inayama; Hirohide (Nara,
JP), Jonokuchi; Hideki (Nara, JP), Mori;
Sadaaki (Mie, JP), Ishihara; Atsushi (Oxford,
GB), Takahashi; Tomofumi (Aichi, JP),
Kitabayashi; Minoru (Aichi, JP), Horie; Tetsuo
(Aichi, JP) |
Assignee: |
Koyo Seiko Co., Ltd. (Osaka,
JP)
|
Family
ID: |
27345988 |
Appl.
No.: |
10/931,421 |
Filed: |
September 1, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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068344 |
Feb 6, 2002 |
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Foreign Application Priority Data
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Feb 14, 2001 [JP] |
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2001-37560 |
Mar 5, 2001 [JP] |
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2001-60989 |
Jun 20, 2001 [JP] |
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2001-185927 |
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Current U.S.
Class: |
310/216.043;
29/596; 310/156.53 |
Current CPC
Class: |
H02K
1/278 (20130101); H02K 29/03 (20130101); H02K
2201/06 (20130101); Y10T 29/49009 (20150115) |
Current International
Class: |
H02K
1/27 (20060101); H02K 29/03 (20060101); H02K
001/00 () |
Field of
Search: |
;310/156.53,216
;29/596 |
References Cited
[Referenced By]
U.S. Patent Documents
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4769567 |
September 1988 |
Kurauchi et al. |
5331245 |
July 1994 |
Burgbacher et al. |
5811904 |
September 1998 |
Tajima et al. |
5936323 |
August 1999 |
Shibukawa et al. |
6049153 |
April 2000 |
Nishiyama et al. |
6188157 |
February 2001 |
Tajima et al. |
6218753 |
April 2001 |
Asano et al. |
6226856 |
May 2001 |
Kazama et al. |
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Foreign Patent Documents
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41 33 723 |
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Apr 1993 |
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DE |
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10016002 |
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Nov 2000 |
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DE |
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08251847 |
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Jan 1997 |
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JP |
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09051643 |
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Jun 1997 |
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JP |
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09182331 |
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Nov 1997 |
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JP |
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11098793 |
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Jul 1999 |
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JP |
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2000116084 |
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Sep 2000 |
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JP |
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2002238231 |
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Aug 2002 |
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JP |
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Primary Examiner: Waks; Joseph
Attorney, Agent or Firm: Darby & Darby
Parent Case Text
This patent application is a divisional of prior U.S. patent
application Ser. No. 10/068,344 Feb. 6, 2002.
Claims
What is claimed is:
1. A brushless DC motor comprising: a stator constructed by
layering plural steel plates, said stator including a yoke on an
outer circumferential portion, plural teeth extending from said
yoke toward a central portion, and notch portions or cavity
portions in an outer circumferential surface of said yoke, wherein
said steel plates are layered while displacing said steel plates at
a predetermined angle in a circumferential direction so that a
length of said notch portions or said cavity portions of each of
said teeth in a layering direction of said steel plates is
substantially equal.
2. The brushless DC motor of claim 1, wherein a substantially equal
number of said steel plates are layered at an equal angle to form
blocks, and said steel plates are layered while displacing said
blocks at a predetermined angle in a circumferential direction.
3. The brushless DC motor of claim 1, wherein said notch portions
or cavity portions are formed in said steel plates for every other
tooth.
4. The brushless DC motor of claim 1, wherein said notch portions
or cavity portions are arranged so that adjacent notch portions or
cavity portions of the angularly displaced steel plates in a cross
sectional view in the layering direction are in point contact with
or separated from each other.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a brushless DC motor comprising a
rotor having permanent magnets, and more particularly relates to a
brushless DC motor capable of reducing cogging torque and a
manufacturing method of the same.
A brushless DC motor is a motor that comprises a rotor having
permanent magnets and rotates the rotor by controlling an electric
commutator circuit for generating a rotational magnetic field in a
stator, based on a detection signal representing the rotational
position of the rotor. Since the brushless DC motor does not
generate mechanical and electrical noises and has high rotary
performance and a long life, it is mainly used in the cylinder of a
VTR, the capstan of a cassette tape deck, a flexible disk driver, a
CD player, etc. In recent years, the brushless DC motor is used in
the drive motor of a power steering apparatus for vehicle.
In the brushless DC motor, torque pulsation, i.e., cogging torque,
is unavoidably produced because of the presence of slots for
winding in the stator and the presence of permanent magnets in the
rotor. The cogging torque is a periodical torque change that is
caused in a motor by a change of magnetic flux owing to the
position of the rotor.
Conventionally, as a method for preventing the cogging torque of a
brushless DC motor, there has been a proposed method for reducing
cogging torque by dividing the rotor into two blocks and combining
the two blocks while displacing the arrangement angle of the rotor
blocks in a circumferential direction so that cogging torques
generated in the two blocks are mutually in antiphase with respect
to the rotation of the rotor.
FIG. 1 is an explanatory view showing an anti-cogging measure taken
by such a rotor, and shows a perspective view of a rotor 105 as an
assembly of an upper-stage rotor block 110 and a lower-stage rotor
block 126. The rotor block 110 comprises an internal rotor core
112, and four permanent magnets 111 attached to the outer
circumference of the rotor core 112 at equal intervals. The rotor
block 120 comprises an internal rotor core 122, and four permanent
magnets 121 attached to the outer circumference of the rotor core
122 at equal intervals.
The rotor blocks 110 and 120 are of the same constructions and
combined in an axial direction while displacing the arrangement
angles by an amount of a mechanical angle .theta.12 at which they
are mutually in antiphase with respect to a pulsation period of
cogging torque generated by the relationship with an opposing
stator. Accordingly, the pulsation components of cogging torques
generated in the rotor blocks 110 and 120 cancel each other out,
thereby reducing the cogging torque of the brushless DC motor.
FIG. 2 is a view showing the relationship between the conventional
rotor blocks 110, 120 and the stator as a cross section in a
direction perpendicular to the rotary shaft. A stator 101 is formed
by layering a number of thin electromagnetic steel plates and
fixing them integrally, and comprises a yoke 102 as an outer
circumferential portion and teeth 103 that are provided at equal
intervals to protrude from the yoke 102 toward the center. Adjacent
teeth 103 form a slot 104 together with the yoke 102. Actually,
armature windings are wound on the teeth 103 and stored in the
slots 104.
FIGS. 3A and 3B are waveform charts for explaining the
above-mentioned anti-cogging torque measure. The vertical axis
indicates cogging torque, while the horizontal axis shows the
rotation angle of the rotor 105. Each of cogging torque Tc1
generated in the upper-stage rotor block 110 and cogging torque Tc2
generated in the lower-stage rotor block 120 has a pulsation period
.theta.11.
In the case where the cogging torques Tc1 and Tc2 are such
sinusoidal waveforms that have the same change in an increasing
direction and a decreasing direction with respect to the center of
amplitude, if the rotor blocks 110 and 120 are combined to have a
phase shift corresponding to a half period .theta.12 of the
pulsation period .theta.11, the pulsation components of the cogging
torques Tc1 and Tc2 of the rotor blocks 110 and 120 cancel each
other out and, ideally, their composite cogging torque Tct is made
a straight waveform having no pulsation as shown in FIG. 3B.
A prerequisite for effectively realizing the above-mentioned method
is that the pulsation waveforms of the cogging torques Tc1 and Tc2
generated in the respective rotor blocks 110 and 120 are mutually
in antiphase in the moving direction of the rotor with respect to
the center of the amplitude and have magnitudes so that the cogging
torques Tc1 and Tc2 cancel each other out. In an actual brushless
DC motor, it is possible to significantly improve the pulsation
period of cogging torque by the above-mentioned method, but there
is a problem that small cogging torque pulsation remains. In order
to improve such small cogging torque and design a brushless DC
motor having no distortion in the rotational operation,
conventionally an abrupt change of cogging torque is prevented and
the pulsation is restricted by widening the gap between the rotor
and the stator to a large extent, providing unequal gaps in the
circumferential direction and intentionally leaking a part of
magnetic flux from the permanent magnets of the rotor near the
region between the magnetic poles. However, the motor efficiency is
sacrificed.
FIGS. 4A and 4B are waveform charts for explaining the influence of
such an anti-cogging torque measure, in which the same codes as in
FIGS. 3A and 3B are used. The pulsation of the cogging torques Tc1
and Tc2 is caused by a change of magnetic flux distribution which
occurs between the permanent magnets 111, 121 and the stator 101
with respect to the rotary direction of the rotor 105. In
particular, the presence of the openings of the slots 104 of the
stator largely affects this change. There is a difference in the
magnetic flux distribution in the magnetic path of the stator 101
and the rotor 105 between the case where the region between the
magnetic poles of the rotor 105 approaches the opening of the slot
104 and the case where the region between the magnetic poles moves
away form the opening. As a result, there is a possibility that the
pulsation of cogging torque does not become a sinusoidal waveform
having the same change in the increasing direction and the
decreasing direction with respect to the center of amplitude.
Moreover, in an ordinary motor structure, the opening of the slot
104 of the stator 101 is narrowed to ensure an interlinkage
magnetic flux from the rotor 105 to the stator 101, and the width
of the tooth 103 is made larger than the width of the opening of
the slot 104 so as to realize high torque and high efficiency.
Therefore, the cogging torque has a small change in a section where
the region between magnetic poles of the rotor 105 faces the tooth
103 during the rotational movement, but a large change in a section
where the region faces the opening of the slot 104. Thus, as shown
in FIG. 4A, waveform distortion including even harmonics
symmetrical about a point may be caused. Even if the pulsations of
such cogging torque waveforms are combined while displacing them by
an amount corresponding to the half period 012 of the pulsation
period 011, there is a problem that the pulsation of cogging torque
as shown in FIG. 4B still remains.
The cogging torque Tct of the rotor obtained by dividing the rotor
into two blocks and layering the blocks while displacing them by an
amount corresponding to the half period .theta.12 of the pulsation
period .theta.11 of cogging torque is shown by expression (1).
Here, T0 is a peak value of a fundamental wave component of cogging
torque when the rotor is not divided, x is an electrical angle of
the angle of an arbitrary rotational position of the rotor, n is a
natural number, and kn is the ratio of the 2n-th harmonics content
to the fundamental wave. ##EQU1##
It is apparent from the expression (1) that, in the brushless DC
motor comprising the rotor divided into two blocks, the fundamental
wave components of cogging torques cancel each other out and are
thus eliminated, but there is a problem that the even harmonics
components remain.
There is another conventional anti-cogging torque measure shown in
FIG. 5, for example. In FIG. 5, the same parts as those shown in
FIG. 2 are designated with the same numbers. In the example shown
in FIG. 5, one with an outer circumference having a curvature
larger than the curvature of the outer circumference of the rotor
core 106 is used as the permanent magnet 107 of the rotor 105.
Moreover the gap between the permanent magnet 107 and the teeth 103
of the stator 101 gradually increases from the center toward the
ends of the permanent magnet 107 in the circumferential direction.
Therefore, when the rotor core 106 rotates, the magnetic flux
interlinking with the teeth 103 changes smoothly instead of
stepwise. Thus, a reduction in cogging torque is made. Some
countermeasure produces a similar effect by changing the shape of
the outer circumference of the rotor core 106 instead of changing
the shape of the permanent magnets 107.
FIG. 6 shows still another conventional anti-cogging torque
measure. In the example shown in FIG. 6, a skew angle .theta.S is
provided for the arrangement of magnetic poles in the axial
direction of the rotor 105. Hence, when the rotor 105 rotates, the
timing in which the boundary between the magnetic poles crosses the
teeth of the stator varies according to a position in the axial
direction of the rotor 105. Thus, the change of the magnetic flux
interlinking with the teeth is made moderate, and the cogging
torque is reduced.
However, both of the conventional techniques shown in FIGS. 5 and 6
suffer from problems including poor magnetic efficiency. In the
technique shown in FIG. 5, since the average gap between the
permanent magnets 107 and the teeth 103 is large, the magnetic
efficiency is poor and a rotary output proportional to the magnetic
force of the permanent magnets 107 can not be obtained. Moreover,
it is necessary to perform various analysis and trial manufacture
to determine the shape of the permanent magnets 107 or the outer
circumference of the rotor core 106, resulting in high development
costs. Furthermore, it is necessary to process the small
configurations accurately, and thus the processing itself is
difficult. Nevertheless, an objective to reduce the cogging torque
is not sufficiently achieved. In particular, when strong rare-earth
based permanent magnets are used to meet the demand for a reduction
in size as in recent years, the cogging torque in itself is
considerably large. Therefore, such a method is not sufficient.
Similarly, the technique shown in FIG. 6 suffers from poor magnetic
efficiency and can not obtain a sufficient rotary output. The
reason for this is that there is the skew angle .theta.S in the
arrangement of magnetic poles and consequently the effective
magnetic flux of the magnetic poles becomes smaller by a
corresponding amount. In the example shown in FIG. 6, one magnetic
pole occupies substantially a parallelogram region on the side face
of the rotor 105. In a portion near the acute apex, the magnetic
flux in the portion does not effectively perform the function of
the motor. Therefore, like the example shown in FIG. 5, this
technique can not obtain a sufficient rotary output.
In recent years, the brushless DC motor is often made to have a
small size and high output by using a rare-earth material, etc. for
the permanent magnets, and tends to be used as a magnetic circuit
in a high magnetic flux density region of the thin electromagnetic
steel plates. On the other hand, there is a problem that the motor
performance is degraded as a result of the promotion of the
reduction in the size of the motor and the generation of extremely
high heat by the motor for the size of the motor. In order to solve
this problem, notch portions are provided in the outer
circumference of the stator and a cool air or the like is caused to
flow through the notch portions to cool the motor and limit the
generation of heat. Besides, in order to achieve another objective
to ensure a punching yield of electromagnetic steel plates and a
gap in the layering direction for sticking means such as welding,
notch portions are provided on the outer circumference side of the
stator.
FIG. 7 is a perspective view showing an example of the stator of a
conventional brushless DC motor having such notch portions. In FIG.
7, the same parts as in those of FIGS. 2 and 5 are designated with
the same numbers.
A notch portion 109 running from the upper end to the lower end of
the stator 101 is provided on the outer circumferential surface of
the yoke 102, at a position near the outside of every third tooth
103. The notch portions 109 are provided on the outer circumference
as the cooling paths for releasing heat during the operation of the
motor and for the purpose of easing the welding that is performed
for fixing plural layered steel plates (by using the protrusions in
the notch portions 109) and easing the punching of material to
improve the yield. By providing the notch portions 109 on the outer
circumference, it is possible to prevent the welded section from
fixing out of the outer circumference of the stator 101 in welding
the thin electromagnetic thin plates to fix them integrally.
Moreover, the notch portions 109 are often provided for the purpose
of saving the material of the thin electromagnetic steel plates of
the stator 101. As described above, each of the notch portions 109
runs from the upper end to the lower end of the stator 101 and has
a length S0 in the layering direction.
In the above-described conventional stator 101, since the notch
portions 109 are aligned with the layering direction, there is a
difference in the magnetic resistance seen from the inside of the
stator 101 between a region of the teeth 103 where the notch
portion 109 is present on the outer circumference side of the
stator 101 and a region where the notch portion 109 is not present.
In the case where a rotor having permanent magnets is positioned
inside the stator 101, a magnetic circuit in which the magnetic
flux flows is formed between the stator 101 and the rotor which
faces the teeth 103 and have permanent magnets arranged so that
adjacent permanent magnetic have opposite polarities. This magnetic
circuit is formed as a magnetic closed circuit composed mainly of
the shortest path between adjacent opposite poles. The shortest
magnetic circuit starting from a region between the magnetic poles
of the permanent magnets of the rotor is most of the causes of
generation of cogging torque. In this magnetic circuit, there is a
big difference in the magnetic flux amount between a region of the
teeth 103 where the notch portion 109 is present on the outer
circumference side of the stator 101 and a region where the notch
portion 109 is not present. Thus, the difference in the magnetic
flux amount according to the positions in the rotary direction of
the rotor is one of the causes of cogging torque, and is a cause of
generation of sound and vibration.
FIG. 8 is a view showing the state of magnetic flux in such a
brushless DC motor. Here, the rotor 105 having the permanent
magnets 107 attached to the surface of the rotor core 106 is
disposed inside the stator 101 shown in FIG. 7. The stator 101 is
formed by layering necessary pieces of thin electromagnetic steel
plates having a portion equivalent to the notch portion 109 on the
outer circumference side of every third portion equivalent to the
tooth 103. Note that the rotor 105 may be a buried-type rotor
having permanent magnets buried in the rotor core 106.
A magnetic flux generated by the relative positional relationship
between the stator 101 and the regions between the magnetic poles
of the opposing permanent magnets 107 of the rotor 105 flows in
respective portions of the stator 101. The magnetic flux amount in
a magnetic path a in a region of the teeth 103 where a notch
portion 109 is present on the outer circumference side of the
stator 101 is denoted as .phi.1, the magnetic flux amount in a
magnetic path b in a region of the teeth 103 where no notch portion
109 is present on the outer circumference side of the stator 101 is
denoted as .phi.2, and the magnetic flux amount in a magnetic path
c in a region where a notch portion 109 different from that for the
flux amount .phi.1 is present is denoted as .phi.3. Here, if the
notch portions 109 have the same configuration, it is clear that
only the difference between the magnetic flux amounts .phi.1 and
.phi.3 is the position of the notch portion 109 in the magnetic
path, and the magnetic flux amounts .phi.1 and .phi.3 are the same
in magnitude.
Here, as shown in FIG. 8, when straight lines A, B and C are drawn
from the center of the shaft hole of the rotor 105 through the
center of the slots 104 toward the outer circumference of the
stator 101, if a region between the magnetic poles of the permanent
magnets 107 of the rotor 105 is positioned on the straight line A,
the magnetic flux from the permanent magnets 107 near the region
between the magnetic poles forms a closed circuit of the magnetic
flux amount .phi.1 by the magnetic path a shown by a dotted line.
Besides, when the rotor 105 rotates clockwise and the region
between the magnetic poles of the permanent magnets 107 reaches the
straight line B, the magnetic flux from the permanent magnets 107
near the region between the magnetic poles forms a closed circuit
of the flux amount .phi.2 by the magnetic path b shown by a dotted
line. When the rotor 105 further rotates clockwise and the region
between the magnetic poles of the permanent magnets 107 reaches the
straight line C, the magnetic flux from the permanent magnets 107
near the region between the magnetic poles forms a closed circuit
of the flux amount .phi.3 by the magnetic path c shown by a dotted
line.
There is a difference in the cross sectional area of the magnetic
path due to the presence and absence of the notch portion 109 in
the magnetic path, between the state where the region between the
magnetic poles of the permanent magnets 107 of the rotor 105 is
positioned on the straight line A and the state where the region
between the magnetic poles is positioned on the straight line B.
Accordingly, there is a difference in the magnetic resistance, and
the flux amounts are .phi.1<.phi.2. Similarly, there is a
difference in the cross sectional area of the magnetic path due to
the presence and absence of the notch portion 109, between the
state where the region between the magnetic poles of the permanent
magnets 107 of the rotor 105 is positioned on the straight line B
and the state where the region between the magnetic poles is
positioned on the straight line C. Accordingly, there is a
difference in the magnetic resistance, and the flux amounts are
.phi.3<.phi.2.
Hence, when the region between the magnetic poles of the permanent
magnets 107 of the rotor 105 is positioned on the straight line B
having no notch portion 109 on the outer circumference side of the
stator 101, the strongest magnetic coupling is obtained between the
rotor 105 and the stator 101. The change in cogging torque
resulting from such phenomena is that the largest cogging torque
appears when the region between the magnetic poles approaches or
moves away from the position of the straight line B because the
magnetic coupling is strong in that position as shown in FIG. 9 and
described above. In FIG. 9, the vertical axis indicates the cogging
torque TC and the horizontal axis shows the rotation angle .theta.
of the rotor 105, and the positions of the straight lines A to C
shown in FIG. 8 correspond to the positions of the straight lines A
to C of FIG. 9.
However, in the brushless DC motor, since the notch portions 109
are provided on the outer circumference side of the stator 101, the
size of the cross sectional area of the magnetic paths varies
because of the difference in the magnetic paths as described above.
As a result, the brushless DC motor suffers from a problem of
deterioration of the pulsation of cogging torque.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a brushless DC
motor capable of effectively eliminating cogging torque.
It is another object of the present invention to provide a
brushless DC motor capable of certainly reducing cogging torque
without making almost no sacrifice to the output
characteristics.
It is still another object of the present invention to provide a
brushless DC motor having notch portions or cavity portions in its
stator and capable of reducing the pulsation of cogging torque.
It is yet another object of the present invention to provide a
method of manufacturing a brushless DC motor, capable of
manufacturing a brushless DC motor of excellent quality.
A brushless DC motor according to the first aspect is a brushless
DC motor comprising: a rotor having plural permanent magnets; and a
stator having plural slots, wherein the rotor is divided into three
rotor blocks in a rotation axis direction, and the three rotor
blocks are layered so that arrangement angles of the rotor blocks
differ from each other by an amount of a mechanical angle in a
rotary direction that is equivalent to one third of a pulsation
period of cogging torque generated by the rotor and stator. It is
thus possible to realize a brushless DC motor capable of
effectively eliminating cogging torque.
A brushless DC motor according to the second aspect is based on the
first aspect, wherein a sum of an effective polar 10 opening angle
of one permanent magnet and a difference between the arrangement
angles of the rotor block located on one end and the rotor block
located on the other end is not more than a pole pitch angle of the
rotor. Thus, each permanent magnet does not overlap adjacent
different magnetic pole, and every magnetic flux from the permanent
magnet becomes an effective magnetic pole. Therefore, in comparison
with a rotor that uses skew magnets and a rotor that uses a ring
magnet magnetized on the skew as anti-cogging measures which are
usually performed in this type of motor, the amount of permanent
magnets to be used can be decreased and the cogging torque can be
reduced without deteriorating the electrical characteristics.
Moreover, it is possible to perform built-in magnetization for
magnetizing the rotor in a non-magnetic state by using the stator
as a magnetic yoke after assembling the motor. Accordingly, it is
possible to eliminate a difficult handling work caused by the rotor
blocks being attracted to the stator, case, etc. and to prevent
dust such as an iron powder attracted to the rotor blocks from
being carried into the case, thereby realizing a brushless DC motor
having good quality.
A brushless DC motor according to the third aspect is a brushless
DC motor comprising: a rotor having plural permanent magnets; and a
stator having plural slots, wherein each of the permanent magnets
is divided into three permanent magnets in a rotation axis
direction, and the three permanent magnets are layered so that
arrangement angles of the permanent magnets differ from each other
by an amount of a mechanical angle in a rotary direction that is
equivalent to one third of a pulsation period of cogging torque
generated by the rotor and stator. It is therefore possible to
realize a brushless DC motor capable of effectively eliminating
cogging torque. Moreover, since this is achieved only by the
arrangement of the permanent magnets without dividing the rotor
core itself, it is possible to realize a brushless DC motor that is
easy to assemble.
A brushless DC motor according to the fourth aspect is based on the
third aspect, wherein a sum of an effective polar opening angle of
one of the permanent magnets and a difference between the
arrangement angles of the permanent magnets located on both ends in
the rotation axis direction among the three permanent magnets is
not more than a pole pitch angle of the rotor. Accordingly, each
permanent magnet does not overlap adjacent different magnetic pole,
and every magnetic flux from the permanent magnets becomes an
effective magnetic pole. Therefore, in comparison with a rotor that
uses skew magnets and a rotor that uses a ring magnet magnetized on
the skew as anti-cogging measures which are usually performed in
this type of motor, the amount of permanent magnets to be used can
be decreased and the cogging torque can be reduced without
deteriorating the electrical characteristics. Moreover, since this
is achieved only by the arrangement of the permanent magnets
without dividing the rotor core itself, it is possible to realize a
brushless DC motor that is easy to assemble.
A brushless DC motor according to the fifth aspect is a brushless
DC motor comprising: a rotor having plural magnetic poles provided
at an equal pitch in a circumferential direction by mounting
permanent magnets in magnet mounting holes; and a stator having
plural slots arranged at an equal pitch in a circumferential
direction, wherein the magnetic poles of the rotor include magnetic
poles whose magnet deviation angle formed by a central line of an
effective polar opening angle and a central line of the magnet
mounting hole is a first angle; and magnetic poles whose magnet
deviation angle is a second angle. Of course, the first angle and
the second angle are different angles. In this brushless DC motor,
the magnetic poles of the rotor include magnetic poles whose magnet
deviation angle is the first angle (hereinafter referred to as the
"magnetic poles of the first angle") and magnetic poles whose
magnet deviation angle is the second angle (hereinafter referred to
as the "magnetic poles of the second angle"). Therefore, the phase
of cogging torque generated by the magnetic pole of the first angle
and the phase of cogging generated by the magnetic pole of the
second angle do not coincide with each other. In other words, these
cogging torques do not reach the peaks simultaneously because of
the following reason. In these magnetic poles, there is a
difference in the timing in which an end of the magnet crosses an
end of the slot of the stator by rotation. For this reason, the
overall cogging torque of the brushless DC motor is reduced as
compared to a construction where all the magnetic poles have an
equal magnet deviation angle.
A brushless DC motor according to the sixth aspect is based on the
fifth aspect, wherein a difference .theta.6 between the second
angle and the first angle is within a range defined by
0.2.times..theta.7.ltoreq..theta.6.ltoreq..theta.5-(0.2.times..theta.7),
where .theta.5 is a slot pitch angle of the stator, and .theta.7 is
a slot opening angle of the stator. In this case, if the magnetic
pole of the first angle is set as a standard, then, in the magnetic
pole of the second angle, the permanent magnet is mounted at a
position preceding (or succeeding) by an amount of the angle
.theta.6 in the rotary direction. Here, if the first angle is zero,
then the second angle is .theta.6. In this case, in the magnetic
pole of the first angle, the central line of the effective polar
opening angle and the central line of the magnet mounting hole
coincide with each other. On the other hand, in the magnetic pole
of the second angle, the central line of the effective polar
opening angle deviates from the central line of the magnet mounting
hole by an amount of .theta.6. Hence, there is a corresponding
difference in the timing in which the cogging torque reaches the
peak between these magnetic poles. If this difference is too small,
the effect of reducing the overall cogging torque of the brushless
DC motor is not sufficient. On the other hand, if the difference is
too large, the timings in which the cogging torques reach the peaks
become close to each other because of the relationship with
adjacent slot of the stator. More specifically, when the magnet
deviation angle is as large as the slot pitch angle .theta.5 of the
stator, such a result is given. When the difference between the
first angle and the second angle is within the above-mentioned
range, such a result is not given and the overall cogging torque of
the brushless DC motor is certainly restricted.
A brushless DC motor according to the seventh aspect is based on
the fifth or sixth aspect, wherein the number of the magnetic poles
of the first angle and the number of the magnetic poles of the
second angle are equal to each other on the rotor. It is therefore
possible to more certainly restrict the overall cogging torque by
the mutual cancellation of the cogging torque waveforms of the
magnetic poles of the first angle and the magnetic poles of the
second angle.
A brushless DC motor according to the eighth aspect is based on any
one of the fifth through seventh aspects, wherein the magnetic pole
of the first angle and the magnetic pole of the second angle are
arranged next to each other on the rotor. Since the rotor is a
rotary member, the rotary balance must be taken into consideration.
Changing the magnet deviation angle by the magnetic poles may shift
the center of gravity of the rotor from the center of the axis and
deteriorate the rotary balance. However, it is possible to minimize
the deterioration of the rotary balance by arranging the magnetic
pole of the first angle and the magnetic pole of the second angle
next to each other.
A brushless DC motor according to the ninth aspect is based on any
one of the fifth through seventh aspects, wherein the rotor is
divided into plural bocks in a rotation axis direction, and the
magnetic pole of the first angle and the magnetic pole of the
second angle are arranged at corresponding positions in the
rotation axis direction in different blocks. The corresponding
positions in the rotation axis direction mean the positions having
the same angular coordinates about the axis. Accordingly, in the
rotor as a whole, the cancellation of the cogging torque waveforms
is achieved within a single magnetic pole. It is therefore possible
to obtain such an effect that as if the cogging torque generated by
a single magnetic pole is reduced. Consequently, the overall
cogging torque of the brushless DC motor is effectively
restricted.
A brushless DC motor according to the tenth aspect is based on the
sixth aspect, wherein the rotor further includes magnetic poles
whose magnet deviation angle is a third angle (hereinafter referred
to as the "magnetic poles of the third angle"), a difference
.theta.6 between the second angle and the first angle is within the
range defined by the above expression, and a difference between the
third angle and the first angle has the same value as and opposite
sign to .theta.6, i.e., -.theta.6. In this case, if the magnetic
pole of the first angle is set as a standard, then, in the magnetic
pole of the second angle, the permanent magnet is mounted at a
position preceding only by an amount of the angle .theta.6 in the
rotary direction. In the magnetic pole of the third angle, the
permanent magnet is mounted at a position succeeding only by an
amount of the angle .theta.6 in the rotary direction. It is also
possible to switch the preceding and succeeding relation.
Accordingly, the cancellation of cogging torques is performed by
three waveforms mutually shifted at equal intervals. Therefore, the
overall cogging torque of the brushless DC motor can be more
certainly restricted.
A brushless DC motor according to the eleventh aspect is based on
the tenth aspect, wherein the number of the magnetic poles of the
first angle, the number of the magnetic poles of the second angle
and the number of the magnetic poles of the third angle are equal
to each other. Therefore, the overall cogging torque of the
brushless DC motor can be more certainly restricted by the
cancellation of the cogging torque waveforms among the magnetic
poles of the first angle, the magnetic poles of the second angle
and the magnetic poles of the third angle.
A brushless DC motor according to the twelfth aspect is based on
the eleventh aspect, wherein a total number of the magnetic poles
of the rotor is an integral multiple of 6, and all of the magnetic
poles of the rotor are any magnetic pole among the magnetic poles
of the first angle, the magnetic poles of the second angle and the
magnetic poles of the third angle. In this construction, since
there is no extra waveform components, it is possible to more
certainly restrict the cogging torque. Note that, in a construction
where the rotor is divided into plural blocks in the axial
direction, the total number of magnetic poles is the product of the
number of blocks and the number of the magnetic poles in each
block.
A brushless DC motor according to the thirteenth aspect is a
brushless DC motor comprising: a rotor having plural magnetic poles
provided at an equal pitch in a circumferential direction by
mounting permanent magnets in magnet mounting holes; and a stator
having plural slots arranged at an equal pitch in a circumferential
direction, wherein the rotor comprises convex portions
corresponding to the magnetic poles on its circumference, and the
magnetic poles of the rotor include magnetic poles whose convex
portion deviation angle formed by a central line of the convex
portion and a central line of the magnet mounting hole is a first
angle; and magnetic poles whose convex portion deviation angle is a
second angle. In this brushless DC motor, the magnetic poles of the
rotor include the magnetic poles whose convex portion deviation
angle is the first angle and the magnetic poles whose convex
portion deviation angle is the second angle. The phases of cogging
torques generated by these magnetic poles do not coincide with each
other. More specifically, these cogging torques do not reach the
peaks simultaneously because of the following reason. In these
magnetic poles, there is a difference in the timing in which an end
of the convex portion crosses an end of the slot of the stator by
rotation. Thus, the overall cogging torque of the brushless DC
motor is reduced as compared to a construction where all the
magnetic poles have an equal convex portion deviation angle.
Moreover, since the convex portions of the rotor is very light,
they have almost no influence on the position of the center of
gravity of the rotor.
A brushless DC motor according to the fourteenth aspect is a
brushless DC motor comprising a rotor having plural magnetic poles
provided at an equal pitch in a circumferential direction by
mounting permanent magnets in magnet mounting holes; and a stator
having plural slots arranged at an equal pitch in a circumferential
direction, wherein the rotor comprises convex portions
corresponding to the magnetic poles on its circumference, and the
magnetic poles of the rotor include magnetic poles whose magnet
deviation angle and convex portion deviation angle are both first
angle; and magnetic poles whose magnet deviation angle and convex
portion deviation angle are both second angle. In this brushless DC
motor, in the magnetic pole where the magnet deviation angle and
convex portion deviation angle are both first angle, the central
line of the effective polar opening angle and the central line of
the convex portion coincide with each other. Similarly, they
coincide with each other in the magnetic pole where the magnet
deviation angle and convex portion deviation angle are both second
angle. Therefore, the magnetic force is efficiently utilized. Of
course, the cogging torque reducing effect by the difference in the
magnet deviation angle and convex portion deviation angle between
the magnetic poles is also obtained.
A brushless DC motor according to the fifteenth aspect is a
brushless DC motor comprising notch portions or cavity portions
provided near an outer circumference side of a part of teeth of a
stator constructed by layering plural steel plates, wherein the
steel plates are layered while displacing the steel plates at a
predetermined angle in a circumferential direction so that a length
of the notch portions or the cavity portions in a layering
direction of each of the teeth of the layered steel plates is
substantially equal. In this brushless DC motor, notch portions or
cavity portions are provided near the outer circumference side of a
part of the teeth of the stator constructed by layering plural
steel plates, and the steel plates are layered while displacing the
steel plates at a predetermined angle in a circumferential
direction so that a length of the notch portions or the cavity
portions in the layering direction of each tooth is substantially
equal. Accordingly, since the difference in the size of the cross
sectional area of the magnetic paths due to the notch portions or
cavity portions can be made smaller, it is possible to realize a
brushless DC motor comprising a stator having notch portions or
cavity portions and capable of reducing the pulsation of cogging
torque.
A brushless DC motor according to the sixteenth aspect is based on
the fifteenth aspect, wherein a substantially equal number of the
steel plates are layered at an equal angle to form blocks, and the
steel plates are layered while displacing the blocks at a
predetermined angle in a circumferential direction. In this
brushless DC motor, the stator can be formed by layering blocks
having aligned notch portions or cavity portions, thereby realizing
a brushless DC motor comprising a stator having notch portions or
cavity portions and capable of reducing the pulsation of cogging
torque.
A brushless DC motor according to the seventeenth aspect is based
on the fifteenth or sixteenth aspect, wherein the notch portions or
cavity portions are formed in the steel plates for every other
tooth. In this brushless DC motor, the difference in the size of
the cross sectional area of the magnetic paths due to the notch
portions or cavity portions can be made smaller and equalized,
thereby realizing a brushless DC motor comprising a stator having
an appropriate number of notch portions and cavity portions and
capable of reducing the pulsation of cogging torque.
A brushless DC motor according to the eighteenth aspect is based on
any one of the fifteenth through seventeenth aspects, wherein the
notch portions or cavity portions are arranged so that adjacent
notch portions or cavity portions of the angularly displaced steel
plates in a cross sectional view in the layering direction are in
point contact with or separated from each other. In this brushless
DC motor, the difference in the size of the cross sectional area of
the magnetic paths due to the notch portions or cavity portions can
be made smaller and equalized, thereby realizing a brushless DC
motor comprising a stator having notch portions or cavity portions
with less magnetic flux leakage and capable of reducing the
pulsation of cogging torque.
A method of manufacturing a brushless DC motor according to the
nineteenth aspect is a method of manufacturing a brushless DC motor
of any one of the first through fourth aspects, wherein the
permanent magnets of the rotor are produced by magnetizing the
rotor blocks or rotor by using the stator as a magnetic yoke after
assembling the motor. Accordingly, it is possible to eliminate a
difficult handling work caused by the permanent magnets being
attracted to the stator, case, etc. and prevent dust such as an
iron powder attracted to permanent magnets from being carried into
the case, thereby enabling the manufacturing of a brushless DC
motor having good quality.
The above and further objects and features of the invention will
more fully be apparent from the following detailed description with
accompanying drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
FIG. 1 is a view showing one example of anti-cogging torque measure
taken by a conventional brushless DC motor;
FIG. 2 is a view showing the relationship between the conventional
rotor blocks and the stator;
FIGS. 3A and 3B are waveform charts for explaining the conventional
anti-cogging torque measure;
FIGS. 4A and 4B are waveform charts for explaining the influence of
the conventional anti-cogging torque measure;
FIG. 5 is a view showing another example of anti-cogging torque
measure taken by the conventional brushless DC motor;
FIG. 6 is a view showing still another example of anti-cogging
torque measure taken by the conventional brushless DC motor;
FIG. 7 is a perspective view showing an example of the stator of
the conventional brushless DC motor;
FIG. 8 is a view for explaining the state of magnetic flux in the
conventional brushless DC motor;
FIG. 9 is a waveform chart showing the change of cogging torque of
the conventional brushless DC motor;
FIG. 10 is a perspective view showing the construction of the rotor
of a brushless DC motor according to the first embodiment of the
present invention;
FIG. 11 is a view showing the displaced state of the rotor
blocks;
FIG. 12A is a waveform chart of cogging torques of three rotor
blocks;
FIG. 12B is a waveform chart of the composite cogging torque of
FIG. 12A;
FIG. 13A is a waveform chart of cogging torques when the cogging
torques of the rotor blocks are being distorted;
FIG. 13B is a waveform chart of the composite cogging torque of
FIG. 13A;
FIGS. 14A, 14B and 14C are views showing the construction of the
rotor of a brushless DC motor according to the second embodiment of
the present invention;
FIG. 15 is a cross sectional view showing partly the construction
of a brushless DC motor according to the third embodiment of the
present invention;
FIG. 16 is a linearly developed view showing the relationship
between the magnetic poles of the rotor and the teeth of the
brushless DC motor of FIG. 15;
FIG. 17 is a waveform chart showing the composite condition of
cogging torque of the brushless DC motor of FIG. 15 in comparison
with the conventional one;
FIG. 18 is a graph showing the relationship between a deviation
angle .theta.6 and the composite cogging torque;
FIG. 19 is a view showing one example of the entire structure of
the rotor of the brushless DC motor of FIG. 15;
FIGS. 20A and 20B are cross sectional views showing the structure
of the rotor of a brushless DC motor according to the fourth
embodiment of the present invention;
FIG. 21 is a cross sectional view showing the structure of the
rotor of a brushless DC motor according to the fifth embodiment of
the present invention;
FIG. 22 is a waveform chart showing the composite condition of
cogging torques of the brushless DC motor of FIG. 21;
FIGS. 23A and 23B are cross sectional views showing the structure
of the rotor of a brushless DC motor according to the sixth
embodiment of the present invention;
FIG. 24 is a cross sectional view showing the structure of the
rotor of a brushless DC motor according to the seventh embodiment
of the present invention;
FIG. 25 is a cross sectional view showing the structure of the
rotor of a brushless DC motor according to the eighth embodiment of
the present invention;
FIG. 26 is a cross sectional view showing a modified example of the
structure of the rotor of the brushless DC motor of the present
invention;
FIG. 27 is a cross sectional view showing another modified example
of the structure of the rotor of the brushless DC motor of the
present invention;
FIG. 28 is a perspective view showing the construction of one
example of the stator of a brushless DC motor according to the
ninth embodiment of the present invention;
FIG. 29 is a side view showing the construction of the stator;
FIG. 30 is a view showing the state of magnetic flux in the
brushless DC motor of the present invention;
FIG. 31 is a view showing the change of cogging torque of the
brushless DC motor of the present invention;
FIG. 32 is a perspective view showing the construction of another
example of the stator of the brushless DC motor according to the
ninth embodiment of the present invention; and
FIG. 33 is a view showing the relationship between the pitch of
teeth and notch portions.
DETAILED DESCRIPTION OF THE INVENTION
The following description will explain the present invention with
reference to the drawings illustrating some embodiments
thereof.
(First Embodiment)
FIG. 10 is a perspective view showing the construction of a rotor
of a brushless DC motor according to the first embodiment of the
present invention. This rotor 5 is constructed by a rotor block 10
in the upper stage, a rotor block 20 in the middle stage, and a
rotor block 30 in the lower stage. The rotor block 10 comprises an
internal rotor core 12, and four permanent magnets 11 attached to
the outer circumference of the rotor core 12 at equal intervals.
The rotor block 20 comprises an internal rotor core 22, and four
permanent magnets 21 attached to the outer circumference of the
rotor core 22 at equal intervals. The rotor block 30 comprises an
internal rotor core 32, and four permanent magnets 31 attached to
the outer circumference of the rotor core 32 at equal
intervals.
The rotor cores 12, 22 and 32 have the same size, and each of which
comprises four coupling member insertion holes 13, 23, 33 for
fixing the rotor blocks 10, 20 and 30 integrally with coupling
members. The permanent magnets 11, 21 and 31 have the same size.
Note that the permanent magnets 11, 21 and 31 are stuck to the
rotor cores 11, 21 and 31, respectively, with an adhesive or the
like. Alternatively, although it is not shown in the drawing, a
protective cover or the like is fixed to the outer circumference of
each of the permanent magnets 11, 21 and 31 by shrinkage fitting,
press fitting or other method. In this case, the material of the
protective cover is suitably selected according to the condition of
use of a non-magnetic material or a magnetic material.
The upper-stage rotor block 10 and the middle-stage rotor block 20
are layered in the axial direction so that they are mutually
displaced at angle .theta.1 in the circumferential direction. The
middle-stage rotor block 20 and the lower-stage rotor block 30 are
layered in the axial direction so that they are mutually displaced
at angle .theta.2 in the opposite direction to the upper-stage
rotor block 10 and the middle-stage rotor block 20. Thus, the
displacement angle between the upper-stage rotor block 10 and the
lower-stage rotor block 30 is the sum of the angle .theta.1 and the
angle .theta.2, and the rotor blocks 10, 20 and 30 are layered
while sequentially displacing them in one direction by rotation.
The mutual displacement angles .theta.1 and .theta.2 of the rotor
blocks 10, 20 and 30 are corresponding to an electrical angle of
120.degree. of a pulsation period of cogging torque (one third of
the pulsation period) generated in the rotor which is not
divided.
FIG. 11 is a view showing the displaced state of each of the rotor
blocks 10, 20 and 30 by the plan views of the respective rotor
blocks 10, 20 and 30. The same parts as in FIG. 10 are designated
with the same numbers. A common central straight line K1 is drawn
through the rotor blocks 10, 20 and 30 to show the displacement
angles of the respective blocks. The rotor block 10 is displaced by
counterclockwise rotation only at the angle .theta.1 formed by the
common central straight line K1 and a central line J1 of the
permanent magnet 11 located on the common central straight line K1.
The rotor block 30 is displaced by clockwise rotation only at the
angle .theta.2 formed by the common central straight line K1 and a
central line J2 of the permanent magnet 31 located on the common
central straight line K1. The rotor block 20 is not displaced, and
the central line of the permanent magnet 21 located on the common
central straight line K1 is positioned on the common central
straight line K1.
These rotor blocks 10, 20 and 30 are aligned with the common
central line K1 and stuck integrally by inserting caulking pines or
bolts, for example, into the coupling member insertion holes 13, 23
and 33. As a result, the rotor blocks 10, 20 and 30 are layered so
that the magnetic pole center positions of the permanent magnets
11, 21 and 31 are displaced sequentially in the circumferential
direction and the positions of the regions between the magnetic
poles of the permanent magnets 11, 21 and 31 are relatively
displaced by the same angle, thereby constructing the rotor.
If such a rotor rotates in the counterclockwise direction, the
pulsation of cogging torque caused by each of the rotor blocks 10,
20 and 30 is such that the pulsation of cogging torque of the rotor
block 10 advances the phase at an electrical angle equivalent to
mechanical angle .theta.1 with respect to the rotor block 20, while
the pulsation of cogging torque of the rotor block 30 delays the
phase at an electrical angle equivalent to mechanical angle
.theta.2 with respect to the rotor block 20.
If the pulsation of cogging torque of each of the rotor blocks 10,
20 and 30 is a sinusoidal wave and the mutual displacement angle of
the rotor blocks 10, 20 and 30 is corresponding to an electrical
angle of 120.degree. of a pulsation period of cogging torque (one
third of the pulsation period) generated in the rotor which is not
divided, then the pulsations of cogging torques of the rotor blocks
10, 20 and 30 are combined in the same manner as three-phase
sinusoidal alternating currents, and cancelled out.
FIG. 12A is a waveform chart of cogging torque of each of the rotor
blocks 10, 20 and 30 in this case and shows the waveforms of
cogging torque Ta in the rotor block 10, cogging torque Tb in the
rotor block 20 and cogging torque Tc in the rotor block 30. In FIG.
12A, 01 indicates a leading phase angle obtained by advancing the
waveform of the cogging torque Tb at an electrical angle equivalent
to the mechanical angle .theta.1 shown in FIG. 11, while 02
indicates a lagging phase angle obtained by delaying the waveform
of the cogging torque Tb at an electrical angle equivalent to the
mechanical angle .theta.1 shown in FIG. 11 (they are designated
with the same codes in FIG. 12A for the purpose of facilitating the
comparison with FIG. 11). In addition, .theta.11 represents the
pulsation period of the cogging torque waveform. FIG. 12B indicates
the composite cogging torque Tcs thereof, and, as explained in
above, pulsation does not appear in the composite cogging torque
Tcs resulting from combining the three-phase sinusoidal waves of
the cogging torques Ta, Tb and Tc.
FIG. 13A is a waveform chart of each cogging torque when distortion
is present in the cogging torques of the rotor blocks 10, 20 and
30. FIG. 13B indicates the composite cogging torque Tcs thereof. In
the case where the cogging torques of the rotor blocks 10, 20 and
30 include even higher harmonics as explained in the conventional
example, if the composite cogging torque Tcs is shown by an
expression similar to expression (1), it is given by the following
expression (2), where x is an electrical angle representing the
angle of an arbitrary rotational position of the rotor and n is a
natural number. According to this expression (2), not only the
fundamental wave components of cogging torques, but also the even
harmonics components thereof cancel each other out, so that
pulsation of cogging torque will not appear theoretically as shown
in FIG. 13B. ##EQU2##
(Second Embodiment)
FIGS. 14A, 14B a 14C show the construction of a rotor of brushless
DC motor according to the second embodiment of the present
invention by a plan view (FIG. 14A) and cross sectional views
(FIGS. 14B and 14C) in a direction perpendicular to the rotary
shaft. This rotor is a permanent magnet-buried type rotor
constructed by inserting permanent magnets into empty holes formed
in a rotor core, and comprises a rotor core 17 and four sets of
permanent magnets fitted at equal intervals into four empty holes
19 formed in the outer circumferential portion of the rotor core 17
at equal intervals along the outer circumference.
Each of the four sets of permanent magnets is composed of three
permanent magnets 16, 26 and 36 located in the upper, middle and
lower stages, the upper stage being one end in the direction of the
rotary shaft. The permanent magnets 16, 26 and 36 have the same
size. The rotor core 17 is formed by layering a number of thin
electromagnetic steel plates and fixing them integrally, and
provided with four coupling member insertion holes 18 for fixing
them integrally with coupling members.
In FIGS. 14A, 14B and 14C, a common central straight line K1 is
drawn through the permanent magnets 16, 26 and 36 to show the
displacement angle of the respective magnets. The upper-stage
permanent magnet 16 is displaced by counterclockwise rotation only
at an angle .theta.1 formed by a central line J1 of the permanent
magnet 16 and the common central straight line K1. The lower-stage
permanent magnet 36 is displaced by clockwise rotation only at an
angle .theta.2 formed by a central line J2 of the permanent magnet
36 and the common central straight line K1. The middle-stage
permanent magnet 26 is not displaced, and the central line of the
permanent magnet 26 coincides with the common central straight line
K1. The mutual displacement angles .theta.1, .theta.2 of the
permanent magnets 16, 26 and 36 are corresponding to an electrical
angle of 120.degree. of a pulsation period of cogging torque (one
third of the pulsation period) generated when the permanent magnets
16, 26 and 36 are not displaced.
Here, the effective polar opening angle of the permanent magnets
16, 26 and 36 is denoted as .theta.m and the pole pitch angle of
the permanent magnets 16, 26 and 36 is denoted as .theta.p. In
order to effectively utilize the present invention, if the
respective values are set so as to satisfy the relationship of the
following expression (3), then at least the effective polar opening
angle .theta.m of the rotor obtained by layering the permanent
magnets 16, 26 and 36 at predetermined displacement angles
according to the point of the present invention will not exceed the
pole pitch angle .theta.p of the permanent magnets 16, 26 and 36.
Therefore, every magnetic flux of the permanent magnets 16, 26 and
36 used in the rotor is utilized as the effective magnetic
flux.
Moreover, as explained in the first embodiment, if the effective
magnetic polar opening angle .theta.m of the rotor obtained by
constructing the rotor blocks according to the point of the present
invention is set so as not to exceed at least the pole pitch angle
.theta.p of the permanent magnets, then the pole pitch angle
.theta.p of the rotor includes the effective polar opening angle
.theta.m of the permanent magnets positioned in the individual
rotor blocks and .theta.1+.theta.2 that is the sum of the
displacement angles of the rotor blocks. Thus, there is no need to
displace the rotor cores in the circumferential direction.
Accordingly, by only displacing the positions of the permanent
magnets divided into three blocks in the rotation axis direction,
in the circumferential direction at a mechanical angle equivalent
to one third of a pulsation period caused by cogging of the rotor
which is not divided, it is possible to obtain the same effects as
the construction where the three divided rotor blocks are layered
while mutually displacing them.
In a type of a rotor shown in FIG. 11 in which permanent magnets
are arranged on the outer circumferential surface of a rotor yoke
(rotor core), a central angle corresponding to the outer
circumferential portion of the effective magnetic pole and a
central angle corresponding to the outer circumferential portion of
the permanent magnet are equal to each other. On the other hand, a
permanent magnet-buried type rotor as shown in FIGS. 14A, 14B and
14C does not especially aim to have a structure for converging the
magnetic flux. However, if the proportion of the magnetic material
portion of the rotor core located between the permanent magnets and
the outer circumference of the rotor is large, the magnetic flux of
the permanent magnet tends to diffuse in this magnetic material
portion. Therefore, the central angle corresponding to the outer
circumferential portion of the effective magnetic pole tends to be
wider than the central angle corresponding to the outer
circumferential portion of the permanent magnet. The central angle
corresponding to the outer circumferential portion of the permanent
magnet can be made smaller by an angle equivalent to this
diffusion. As a result, it is possible to set a larger space
between the permanent magnets in the circumferential direction.
Conventionally, for this type of the rotor, skew-shaped permanent
magnets or ring-shaped permanent magnets magnetized on the skew are
used as cogging torque preventing means. The magnetization of such
a permanent magnet is carried out by skew magnetization using a
special magnetic yoke before incorporating the rotor into a case or
the like. In a brushless DC motor according to the present
invention, the central angle .theta.m corresponding to the outer
circumferential portion of the effective magnetic pole of each
rotor block and the displacement angle of each rotor block are set
so as not to exceed the central angle .theta.p corresponding to the
circumferential interval between the magnetic poles of the rotor.
Therefore, after assembling the motor, the rotor blocks or the
rotor can be magnetized by causing a direct current to flow in the
stator and using the stator as the magnetic yoke. It is thus
possible to achieve built-in magnetization of magnetizing the rotor
in a non-magnetic state.
Note that the above-described first and second embodiments are
illustrated on condition that the rotor or the permanent magnets
are equally divided, but, if the changes in cogging torques of the
divided rotor blocks or the rotor portions of the divided permanent
magnets are substantially the same, it is not necessarily to divide
the rotor and the permanent magnets equally, and it is needless to
say that the materials of the permanent magnets can be different
from each other. Further, while the first and second embodiments
are illustrated with reference to a rotor constructed by burying
permanent magnets as shown in FIGS. 14A, 14B, 14C near its surface
along the outer circumference of the rotor core, it is needless to
explain that the present invention is also applicable to rotors
constructed by burying permanent magnets in concave, V-, reversed
circular arc- or flat plate-shape in the rotor core.
(Third Embodiment)
As shown in FIG. 15, the third embodiment is implemented by
applying the present invention to a brushless DC motor comprising a
magnet-buried type rotor 5 having four magnetic poles. In other
words, the rotor 5 of this brushless DC motor has magnet mounting
holes 8 formed at four positions along the outer circumference, and
permanent magnets 7 are mounted in the magnet mounting holes 8,
respectively. The permanent magnets 7 are arranged adjacent to the
outer circumference of the rotor 5. For this reason, it can be said
that the opening angle of each permanent magnet 7 seen from a
rotational center axis O is equal to the effective polar opening
angle. The permanent magnets 7 are arranged so that adjacent
permanent magnets 7 have mutually opposite polarities. A stator 1
comprises a yoke 2 as an outer circumferential portion, and twelve
teeth 3 that are provided at equal intervals and protrude from the
yoke 2 toward the center. Adjacent teeth 3 form a slot 4 together
with the yoke 2. Note that only the upper half portion is shown in
FIG. 15 for simplification, but the lower half portion also has the
same construction. Further, although not shown in FIG. 15, the
teeth 3 are provided with windings in actual use.
In FIG. 15, J3 and J4 indicate the central lines of the magnet
mounting holes 8, i.e., the pole central lines. They are owing to
the core structure of the rotor 5. K3 and K4 indicate the central
lines of the permanent magnets 7a and 7b, respectively, ie., the
central lines of the effective polar opening angles of the
respective magnetic poles. They represent the center position of
actual magnetic force. .theta.3 indicates the pitch angle owing to
the structure of the rotor 5 seen from the rotational center axis
O. .theta.4 indicates the effective polar opening angle of the
permanent magnet 7a. .theta.5 indicates the slot pitch angle of the
stator 1. .theta.6 indicates the deviation angle between the pole
central line J3 of the magnetic pole of the permanent magnet 7a and
the central line K3 of the effective polar opening angle. .theta.7
indicates the slot opening angle that is the opening angle between
adjacent teeth 3. Here, one of the lines defining both ends of the
slot pitch angle .theta.5 coincides with the central line K3 of the
effective polar opening angle, but this is merely an accident and
has no special meaning. Similarly, it is merely an accident that
the other of the lines defining both ends of the slot pitch angle
.theta.5 coincides with one of the lines defining both ends of the
effective polar opening angle .theta.4. Furthermore, it is merely
an accident that one of the lines defining both ends of the slot
opening angle .theta.7 coincides with one of the lines defining
both ends of the effective polar opening angle .theta.4.
According to the brushless DC motor shown in FIG. 15, in the
magnetic pole of the permanent magnet 7a, there is an angular
deviation between the pole central line J3 and the central line K3
of the effective polar opening angle, and the value of the
deviation angle is .theta.6. On the other hand, in the magnetic
pole of the permanent magnet 7b, the pole central line J4 and the
central line K4 of the effective polar opening angle coincide with
each other. In other words, the value of the deviation angle
between them is zero. Thus, the two magnetic poles have a
difference of .theta.6 in the deviation angle between the pole
central line and the central line of the effective polar opening
angle.
Referring to FIGS. 16 and 17, the following description will
explain the state of cogging torque of the brushless DC motor
having the above-mentioned construction. FIG. 16 shows the teeth 3
(3-1, 3-2, . . . , 3-n, 3-(n+1)), the magnet mounting holes 8 and
the permanent magnets 7b and 7a of the brushless DC motor of FIG.
15 by linearly developing them. FIG. 17 shows the waveforms of
cogging torques generated by the brushless DC motor. The upper row
is a graph when the deviation angle .theta.6 is equal to the slot
opening angle .theta.7 in the brushless DC motor of FIG. 15. The
lower row is shown for a comparison purpose, and is a graph when
the deviation angle .theta.6 is zero. Note that since
.theta.4=.theta.5, in order to establish the relation
0.2.times..theta.7.ltoreq..theta.6.ltoreq..theta.5-(0.2.times..theta.7),
it is necessary to satisfy .theta.7.ltoreq.(5/6).theta.5. However,
since .theta.7 is usually equal to about a half of .theta.5 or
less, the above expression would be established.
Let consider how the permanent magnets 7b and 7a move from the
state shown in FIG. 16 toward the right direction in FIG. 16 with
the rotation of the rotor. At this time, the left end of the
permanent magnet 7b receives an attraction force from the tooth
3-1. This attraction force acts as interference to the movement of
the permanent magnet 7b in the right direction. Then, when the left
end of the permanent magnet 7b reaches a position closer to the
tooth 3-2 than to the tooth 3-1, the left end of the permanent
magnet 7b receives a force in a direction of assisting the
movement. The reason for this is that the attraction force from the
tooth 3-2 becomes dominant. Thus, the left end of the permanent
magnet 7b generates cogging torque that repeats interference and
assistance periodically with respect to the movement.
Similarly, the left end of the permanent magnet 7a generates
cogging torque. However, there is a time difference between a
timing in which the left end of the permanent magnet 7a receives a
force in an interfering direction because of the relationship with
the tooth 3-n and a timing in which the left end of the permanent
magnet 7b receives a force in the interfering direction because of
the relationship with the tooth 3-1. The time difference is a lag
corresponding to the deviation angle .theta.6. There is also a time
difference between the timings of receiving the force in the
assisting direction. Hence, there is a phase angle equivalent to
the deviation angle .theta.6 between the cogging torque generated
by the left end of the permanent magnet 7b and the cogging torque
generated by the left end of the permanent magnet 7a.
The graph in the upper row of FIG. 17 shows this state when the
deviation angle .theta.6 is equal to the slot opening angle
.theta.7. In other words, the thin solid line of this graph
indicates cogging torque TC1 generated by the left end of the
permanent magnet 7b. The broken line indicates cogging torque TC2
generated by the left end of the permanent magnet 7a. Further, the
thick solid line indicates their composite cogging torque TC0. The
amplitude of the cogging torque TC0 is much smaller than the sum of
the amplitudes of the cogging torques TC1 and TC2. The reason for
this is that there is a time difference between the peaks of the
cogging torques TC1 and TC2 due to the phase difference
therebetween and most of the torques cancel each other out.
If this state is compared with the graph in the lower row of FIG.
17 (obtained when the deviation angle .theta.6 is zero), the effect
can be clearly understood. In other words, in the graph of the
lower row, the amplitude of the cogging torque TC0 is equal to the
sum of the amplitudes of the cogging torques TC1 and TC2 (which are
equal to each other in the lower row of FIG. 17). There is no phase
difference between the cogging torques TC1 and TC2, and thus the
cogging torques TC1 and TC2 have the peaks simultaneously and the
torques are added together as it is. In other words, the effect of
the third embodiment is that the amplitude of the cogging torque
TC0 in the graph of the upper row is much smaller than the
amplitude of the cogging torque TC0 in the graph of the lower
row.
Note that this type of brushless DC motor is generally designed so
as to make the slot opening angle .theta.7 as small as possible.
The reason for this is to make the magnetic resistance between the
permanent magnets and the teeth as small as possible and ensure a
large interlinkage magnetic flux. On the other hand, this makes the
change rate (the maximum inclination of the curves shown in the
graph of FIG. 17) of the individual cogging torques (corresponding
to TC1 and TC2) generated by the respective permanent magnets
steep. Here, if the deviation angle .theta.6 between the permanent
magnets 7b and 7a is equal to the slot opening angle .theta.7, the
abrupt changes of the individual cogging torques are effectively
cancelled out. Therefore, the composite cogging torque
(corresponding to TC0) becomes smaller. This is the reason why the
upper row of FIG. 17 shows the example when .theta.6=.theta.7.
Further, if the slot opening angle .theta.7 is equal to one second
of the slot pitch angle .theta.5, the cogging torques TC1 and TC2
establish an antiphase relationship. In this case, the two cogging
torques cancel each other out extremely well.
FIG. 18 is a graph showing the relationship between the deviation
angle .theta.6 and the peak value of the composite cogging torque.
In this graph, the horizontal axis indicates the deviation angle
.theta.6, and the deviation angle .theta.6 is within a range from
zero to the slot pitch angle .theta.5. If the deviation angle
.theta.6 is equal to the slot pitch angle .theta.5, since the
permanent magnet deviates by an amount corresponding to one slot,
this state is the same as a state where there is no deviation. The
vertical axis indicates the pitch value of the composite cogging
torque. The scale on the vertical axis shows a reference value by
denoting a value obtained when the deviation angle .theta.6 is zero
as 1. Moreover, in FIG. 18, two curves TC0-1 (solid line) and TC0-2
(alternate long and short dash line) are drawn. These two curves
resulted from the difference between the waveforms of the
individual cogging torques (corresponding to TC1 and TC2). The
curve TC0-1 represents the state where there is a difference
between the waveform at the time of rising and the waveform at the
time of falling, i.e., even harmonics components are included. The
curve shown in FIG. 17 belongs to this state though it is no so
apparent. In an actual fact, this state occurs more frequently. The
curve TC0-2 represents the state where the waveform at the time of
rising and the waveform at the time of falling are symmetrical,
i.e., even higher harmonics components are not included.
The following will be understood from the graph of FIG. 18.
Regarding the curve TC0-1, at both ends (.theta.6=0,
.theta.6=.theta.5), the value on the vertical axis is 1. The reason
for this is that there is no deviation. At a slightly inward
position in the direction of the horizontal axis, the value on the
vertical axis abruptly decreases. This occurs because of the
cancellation effect of cogging torques caused by the deviation.
After coming to a position where the distances from both ends are
0.2 times the slot opening angle .theta.7
(.theta.6=0.2.times..theta.7,
.theta.6=.theta.5-(0.2.times..theta.7)), the change of the value on
the vertical axis becomes moderate at further inward positions. The
reason for this is that the cogging torques do not completely
cancel each other out because of the difference between the
waveform at the time of rising and the waveform at the time of
falling. Within this range, the value on the vertical axis has a
minimum at the center in the direction of the horizontal axis
(.theta.6=0.5.times..theta.5), that is, substantially a half of the
values at both ends. Thus, it can be understood that the range of
the deviation angle .theta.6 for effectively obtaining the cogging
torque reducing effect is
0.2.times..theta.7.ltoreq..theta.6.ltoreq..theta.5-(0.2.times..theta.7).
Moreover, in view of the curve TC0-2 in the graph of FIG. 18, there
is no big difference from the curve TC0-1 in the vicinity of both
ends. However, even within the range of
0.2.times..theta.7.ltoreq..theta.6.ltoreq..theta.5-(0.2.times..theta.7),
the value on the vertical axis further decreases considerably. At
the center (.theta.6=0.5.times..theta.5), the value on the vertical
axis becomes almost zero. The reason for this is that the cogging
torques are more certainly cancelled out because the waveform at
the time of rising and the waveform at the time of falling are
symmetrical.
In the above-described manner, the overall cogging torque of the
brushless DC motor of FIG. 15 is reduced by the deviation angle
.theta.6. Moreover, in the brushless DC motor of FIG. 15, a
magnetic pole (permanent magnet 7b) with a deviation angle of zero
and a magnetic pole (permanent magnet 7a) with a deviation angle of
.theta.6 are arranged next to each other. Therefore, even though
the permanent magnet 7a has the deviation angle .theta.6, the
center of gravity of the rotor 5 does not much deviate from the
center of the axis. Therefore, the rotary balance of the rotary
object is not affected very much. Thus, the rotation is performed
smoothly.
Note that while FIG. 15 shows the upper half portion of the rotor
5, it is more preferable to arrange one magnetic pole with a
deviation angle of zero and one magnetic pole with the deviation
angle .theta.6 in its lower half portion in the same manner as in
the upper half portion as shown in FIG. 19. Accordingly, two
magnetic poles with a deviation angle of zero and two magnetic
poles with the deviation angle .theta.6 are present in the rotor 5
as a whole, i.e., the number of the respective magnetic poles are
equal to each other. Therefore, the reduction in cogging torque of
the brushless DC motor as a whole is achieved more satisfactorily.
Further, in the brushless DC motor shown in FIG. 19, magnetic poles
with a deviation angle of zero and magnetic poles with the
deviation angle .theta.6 are arranged at symmetrical positions.
Accordingly, in the circumferential direction, the magnetic pole
with a deviation angle of zero and the magnetic pole with the
deviation angle .theta.6 are present alternately. Thus, the
influence of the deviation angle on the position of the center of
gravity of the rotor 5 is cancelled. Consequently, the center of
gravity coincides with the center of the axis, and the rotary
balance is very well.
(Fourth Embodiment)
The fourth embodiment adopts a block construction in which the
rotor is divided in the axial direction. Here, an example in which
the rotor is divided into two blocks will be explained. The rotor
of the fourth embodiment comprises a block shown in FIG. 20A and a
block shown in FIG. 20B. The number of magnetic poles in each block
is 4, that is the same as in FIG. 15. Note that since the stator is
the same as that shown in FIG. 15, it is omitted in FIGS. 20A and
20B.
In the block of FIG. 20A, the deviation angle in each magnetic pole
is as follows. In the magnetic pole (permanent magnet 711) shown at
the upper left in FIG. 20A, there is a deviation of angle .theta.6
between a pole central line J3-1 and an effective polar opening
angle central line K3-1. In the magnetic pole (permanent magnet
712) shown at the upper right in FIG. 20A, a pole central line J4-1
and an effective polar opening angle central line K4-1 coincide
with each other. In the magnetic pole (permanent magnet 713) shown
at the lower right in FIG. 20A, like the upper left magnetic pole,
there is a deviation of angle .theta.6 between a pole central line
J5-1 and an effective polar opening angle central line K5-1. In the
magnetic pole (permanent magnet 714) shown at the lower left in
FIG. 20A, like the upper right magnetic pole, a pole central line
J6-1 and an effective polar opening angle central line K6-1
coincide with each other. In short, two magnetic poles with a
deviation angle of zero and two magnetic poles with the deviation
angle .theta.6 are present, and they are arranged alternately in
the circumferential direction.
In the block of FIG. 20B, the deviation angle in each magnetic pole
is as follows. In the magnetic pole (permanent magnet 721) shown at
the upper left in FIG. 20B, a pole central line J3-2 and an
effective polar opening angle central line K3-2 coincide with each
other. In the magnetic pole (permanent magnet 722) shown at the
upper right in FIG. 20B, there is a deviation of angle .theta.6
between a pole central line J4-2 and an effective polar opening
angle central line K4-2. In the magnetic pole (permanent magnet
723) shown at the lower right in FIG. 20B, like the upper left
magnetic pole, a pole central line J5-2 and an effective polar
opening angle central line K5-2 coincide with each other. In the
magnetic pole (permanent magnet 724) shown at the lower left in
FIG. 20B, there is a deviation of angle .theta.6 between a pole
central line J6-2 and an effective polar opening angle central line
K6-2 like the upper right magnetic pole. In short, two magnetic
poles with a deviation angle of zero and two magnetic poles with
the deviation angle .theta.6 are present, and they are arranged
alternately in the circumferential direction. This is the same as
the previously illustrated structure in FIG. 19.
It can be seen by comparing these two blocks that the presence and
absence of a deviation angle are the opposite between the
corresponding magnetic poles in the axial direction. Therefore, the
brushless DC motor of the fourth embodiment produces the following
effects. Precisely, in each of the two blocks, as explained with
reference to FIG. 19 above, the cogging torque is satisfactorily
reduced and an excellent rotary balance is obtained. In addition,
since the presence and absence of a deviation angle differ between
the corresponding magnetic poles in the axial direction, the
following effects are obtained. Namely, in the rotor as a whole,
within a single magnetic pole (for example, the permanent magnet
711 and the permanent magnet 721, etc.), the cogging torques cancel
each other out. Therefore, in the brushless DC motor as a whole,
the individual cogging torques are respectively reduced.
Consequently, the overall cogging torque is extremely small.
Moreover, regarding the rotary balance, the influence of he
deviation is locally reduced at respective positions of the rotor.
Therefore, this brushless DC motor is suitable for applications for
abrupt acceleration or abrupt deceleration and also an application
used in high-speed rotation. Further, in the corresponding magnetic
poles in the axial direction, the magnetic poles of the permanent
magnets are equal to each other. They include magnetic poles having
a deviation and magnetic pole havings no deviation. For this
reason, in the brushless DC motor as a whole, the effective polar
opening angle is substantially widened. Moreover, the pole pitch of
the rotor itself is an equal interval. Furthermore, in the gap
between the rotor and the stator, the change in the magnetic flux
density in the circumferential direction is smooth.
(Fifth Embodiment)
In the fifth embodiment, the number of magnetic poles in the
circumferential direction of the rotor is made six. The rotor of
the fifth embodiment has the construction shown in FIG. 21. In
other words, a rotor 51 has magnet mounting holes 81 to 86 at six
positions along the outer circumference, and permanent magnets 71
to 76 are mounted in the magnet mounting holes 81 to 86,
respectively. The permanent magnets 71 to 76 are arranged so that
the magnetic poles of adjacent permanent magnets are opposite to
each other. Note that since there is no particular difference
between the stator of this embodiment and the one shown in FIG. 15,
the stator is omitted in FIG. 21.
In the rotor 61, the deviation angle in each magnetic pole is as
follows. In the magnetic pole (permanent magnet 71) shown on the
left side in FIG. 21, a pole central line J11 and an effective
polar opening angle central line K11 coincide with each other. In
the magnetic pole (permanent magnet 72) shown at the upper left in
FIG. 21, there is a deviation of angle .theta.6 between a pole
central line J12 and an effective polar opening angle central line
K12. If the rotary direction of the rotor 51 is clockwise, then the
line K12 is located on a position ahead the line J12 by an amount
corresponding to the angle .theta.6. In the magnetic pole
(permanent magnet 73) shown at the upper right in FIG. 21, there is
a deviation of angle .theta.6 between a pole central line J13 and
an effective polar opening angle central line K13. However, the
direction of the deviation is opposite to that in the upper left
magnetic pole, and thus the line K13 is located on a position
behind the line J13 by an amount corresponding to the angle
.theta.6. In the magnetic pole (permanent magnet 74) shown on the
right side in FIG. 21, a pole central line J14 and an effective
polar opening angle central line K14 coincide with each other like
the left side magnet pole. In the magnetic pole (permanent magnet
75) shown at the lower right in FIG. 21, there is a deviation of
angle .theta.6 between a pole central line J15 and an effective
polar opening angle central line K15 in the leading direction like
the upper left magnetic pole. In the magnetic pole (permanent
magnet 76) shown at the lower left in FIG. 21, there is a deviation
of angle .theta.6 between a pole central line J16 and an effective
polar opening angle central line K16 in the lagging direction like
the upper right magnetic pole.
In short, two magnetic poles with a deviation angle of zero, two
magnetic poles with the deviation angle .delta. in the leading
direction, and two magnetic poles with the deviation angle .theta.6
in the lagging direction are present, and thus the number of the
respective magnetic poles are the same. Magnetic poles having a
deviation angle other than these three deviation angles do no
exist. Further, magnetic poles having a deviation angle in mutually
opposite directions are arranged on both sides (the left and right
sides) of a magnetic pole having a deviation angle of zero.
In the brushless DC motor comprising the rotor 51 of the
above-described structure, the cancellation of cogging torques is
performed as shown in the waveform chart of FIG. 22. In the
waveform chart of FIG. 22, the thin solid line indicates cogging
torque TC11 generated by the left and right magnetic poles in FIG.
21. The broken line indicates cogging torque TC12 generated by the
upper right and lower left magnetic poles in FIG. 21. The alternate
long and short dash line indicates cogging torque TC13 generated by
the upper left and lower right magnetic poles in FIG. 21. Moreover,
the thick solid line indicates composite cogging torque TC10
resulting from combining them. Note that FIG. 21 shows the
waveforms when the magnitude of the deviation angle .theta.6 is
equal to one third of the slot pitch angle .theta.5. Thus, in the
fifth embodiment, the cancellation of cogging torques is carried
out by three-phase composition. Therefore, the composite cogging
torque TC10 shown in FIG. 21 is almost zero. Even when the
individual cogging torques are asymmetrical waveforms containing
even higher harmonics components, the overall cogging torque is
very satisfactorily reduced. Moreover, even when there is slight
distortion in the waveforms or there is a slight variation in the
magnitude of the individual cogging torques, the effect is
stable.
Here, if the rotor 51 of FIG. 21 is seen carefully, it can be
understood that three adjacent magnetic poles in the
circumferential direction can never include magnetic poles having
the same deviation angle (including the direction). This fact is
established for any three adjacent magnetic poles. In the rotor 51,
therefore, the influence of the deviation on the rotary balance is
locally eliminated in the respective positions. Consequently, the
brushless DC motor of the fifth embodiment has excellent rotary
performance.
(Sixth Embodiment)
The sixth embodiment shown in FIGS. 23A and 23B is a combination of
the fourth and fifth embodiments. In other words, the sixth
embodiment comprises six magnetic poles and employs a divided-block
structure. A rotor of the sixth embodiment comprises a block shown
in FIG. 23A and a block shown in FIG. 23B. Note that the
illustration of the stator is also omitted in the sixth
embodiment.
In the block of FIG. 23A, the deviation angle in each magnetic pole
is as follows. In the magnetic pole (permanent magnet 71-1) shown
on the left side in FIG. 23A, a pole central line J11-l and an
effective polar opening angle central line K11-1 coincide with each
other. In the magnetic pole (permanent magnet 72-1) shown at the
upper left in FIG. 23A, there is a deviation of angle .theta.6 in
the leading direction between a pole central line J12-1 and an
effective polar opening angle central line K12-1. In the magnetic
pole (permanent magnet 73-1) shown at the upper right in FIG. 23A,
there is a deviation of angle .theta.6 in the lagging direction
between a pole central line J13-1 and an effective polar opening
angle central line K13-l. In the magnetic pole (permanent magnet
74-1) shown on the right side in FIG. 23A, a pole central line
J14-1 and an effective polar opening angle central line K14-1
coincide with each other. In the magnetic pole (permanent magnet
75-1) shown at the lower right in FIG. 23A, like the upper left
magnetic pole, there is a deviation of angle .theta.6 in the
leading direction between a pole central line J15-1 and an
effective polar opening angle central line K15-1. In the magnetic
pole (permanent magnet 76-1) shown at the lower left in FIG. 23A,
like the upper right magnetic pole, there is a deviation of angle
.theta.6 in the lagging direction between a pole central line J16-1
and an effective polar opening angle central line K16-1. This is
the same as that shown in FIG. 21.
In the block of FIG. 23B, the deviation angle in each magnetic pole
is as follows. In the magnetic pole (permanent magnet 71-2) shown
on the left side in FIG. 23B, there is a deviation of angle
.theta.6 in the lagging direction between a pole central line J11-2
and an effective polar opening angle central line K11-2. In the
magnetic pole (permanent magnet 72-2) shown at the upper left in
FIG. 23B, a pole central line J12-2 and an effective polar opening
angle central line K12-2 coincide with each other. In the magnetic
pole (permanent magnet 73-2) shown at the upper right in FIG. 23B,
there is a deviation of angle .theta.6 in the leading direction
between a pole central line J13-2 and an effective polar opening
angle central line K13-2. In the magnetic pole (permanent magnet
74-2) shown on the right side in FIG. 23B, like the left side
magnetic pole, there is a deviation of angle .theta.6 in the
lagging direction between a pole central line J14-2 and an
effective polar opening angle central line K14-2. In the magnetic
pole (permanent magnet 75-2) shown at the lower right in FIG. 23B,
like the upper left magnetic pole, a pole central line J15-2 and an
effective polar opening angle central line K15-2 coincide with each
other. In the magnetic pole (permanent magnet 76-2) shown at the
lower left in FIG. 23B, like the upper right magnetic pole, there
is a deviation of angle .theta.6 in the leading direction between a
pole central line J16-2 and an effective polar opening angle
central line K16-2.
It can be seen by comparing these two blocks that the corresponding
magnetic poles in the axial direction absolutely have different
deviation angles. Therefore, the brushless DC motor of the sixth
embodiment produces the following effects. Namely, in these two
blocks, as explained with reference to FIG. 21 (the fifth
embodiment) above, the cogging torque is satisfactorily reduced and
an excellent rotary balance is achieved. In addition, the effects
explained in FIGS. 20A and 20B (the fourth embodiment) are also
obtained because of the difference in the deviation angle between
the corresponding magnetic poles in the axial direction. In other
words, in the rotor as a whole, the cancellation of cogging torques
and rotary unbalance is performed even within a single magnetic
pole. Consequently, the overall cogging torque is extremely small,
and the rotary balance is excellent.
(Seventh Embodiment)
In the seventh embodiment, instead of providing a deviation in the
positions of permanent magnets, cancellation of cogging torques
between the magnetic poles is achieved by other means. The other
means is implemented by providing convex portions corresponding to
the magnetic poles on the periphery of the rotor core and shifting
the position of the convex portion in each magnetic pole. A rotor
52 of the seventh embodiment has the construction shown in FIG. 24.
The rotor 52 has four magnetic poles. However, unlike the third
through sixth embodiments, each permanent magnet is mounted in the
center of each magnet mounting hole. In other words, in every
magnetic pole, the pole central line (J21, etc.) and the effective
polar opening angle central line (K21, etc.) coincide with each
other.
However, the rotor 52 has convex portions 61 to 64 corresponding to
the respective magnetic poles on its outer circumference. In FIG.
24, L1 to L4 represent the central lines of the respective convex
portions 61 to 64 seen from the rotational center axis O. Moreover,
in the convex portion 61 shown at the upper left in FIG. 24, the
convex portion central line L1 coincides with a pole central line
J21 and an effective polar opening angle central line K21. In the
convex portion 62 shown at the upper right in FIG. 24, the convex
portion central line L2 is located on a position deviated from a
pole central line J22 and an effective polar opening angle central
line K22, and the deviation angle is .theta.6. In the convex
portion 63 shown at the lower right in FIG. 24, like the convex
portion 61, the convex portion central line L3 coincides with a
pole central line J23 and an effective polar opening angle central
line K23. In the convex portion 64 shown at the lower left in FIG.
24, like the convex portion 62, the convex portion central line L4
is located on a position deviated at the angle .theta.6 from a pole
central line J24 and an effective polar opening angle central line
K24. The convex portions 61 to 64 in FIG. 24 are drawn in an
exaggerated manner to facilitate understanding, and, in an actual
fact, the difference in level caused by the convex portions is very
small.
In a brushless DC motor using the rotor 52, in each magnetic pole,
the gap between the rotor and the stator is smaller and the
magnetic resistance is smaller in a position where the convex
portion is present than in the outside of this position. Therefore,
magnetic flux converges on this position and the positions
including the convex portions dominantly contribute to the
generation of cogging torque. Thus, similarly to explanation given
in the third embodiment, etc., the effect of reducing the cogging
torque is produced by the presence of the convex portions having a
deviation angle and the convex portions having no deviation
angle.
Moreover, in the seventh embodiment, since no deviation is
introduced in the permanent magnets, the following effects are also
produced. Precisely, the influence of the deviation of the convex
portions on the position of the center of gravity of the rotor is
much smaller than that of the permanent magnets. Therefore, an
extremely good rotary balance is obtained. Furthermore, the absence
of deviation in the permanent magnets allows the use of the largest
possible permanent magnet within the range of the magnet mounting
hole. In addition, it is also possible to allow the permanent
magnet to fully occupy the space within the pole pitch. In this
case, stronger rotary force is obtained. Even if such a large
permanent magnet is not used, the seventh embodiment has a merit
that the degree of freedom in designing the brushless DC motor is
high. Besides, the cogging torque generated by a single magnetic
pole is also smaller compared to one without convex portion. The
reason for this is that an end of a permanent magnet and an end of
a convex portion generate cogging torques, respectively, and there
is a phase difference between them.
(Eighth Embodiment)
In the eighth embodiment, deviations are introduced in both of the
permanent magnets and the convex portions. A rotor 53 of the eighth
embodiment is constructed as shown in FIG. 25. In the rotor 53, the
deviation angle in each magnetic pole is as follows. In the
magnetic pole (convex portion 611) shown at the upper left in FIG.
25, all of a pole central line J31, an effective polar opening
angle central line K31 and a convex portion central line L11
coincide with each other. In the magnetic pole (convex portion 612)
shown at the upper right in FIG. 25, both of an effective polar
opening angle central line K32 and a convex portion central line
L12 are located on a position deviated from a pole central line J32
at the angle .theta.6. In the magnetic pole (convex portion 613)
shown at the lower right in FIG. 25, like the upper left magnetic
pole, all of a pole central line J33, an effective polar opening
angle central line K33 and a convex portion central line L13
coincide with each other. In the magnetic pole (convex portion 614)
shown at the lower left in FIG. 25, like the upper right magnetic
pole, both of an effective polar opening angle central line K34 and
a convex portion central line L14 are located on a position
deviated from a pole central line J34 at the angle .theta.6. In the
(upper right and lower left) magnetic poles having a deviation, the
deviation angle of the convex portion and the deviation angle of
the permanent magnet are both .theta.6. Therefore, in any magnetic
pole, the convex portion is positioned at the center of the
permanent magnet.
In the rotor 53, a reduction in cogging torque by the magnetic
poles having a deviation and the magnetic poles having no deviation
is achieved by both of the permanent magnets and the convex
portions. Moreover, the coincidence of the center of the permanent
magnet and the center of the convex portion in each magnetic pole
produces the following effects. First, the magnetic force of the
permanent magnets is more effectively utilized. This effect is
produced by the presence of the convex portion at the center of the
permanent magnet. Here, it is also possible to cancel the cogging
torques, depending on the relationship between the slot pitch angle
and the angular difference between an end of the permanent magnet
and an end of the convex portion.
Modified Examples of Third through Eighth Embodiments)
Next, the following description will explain modified examples of
the configurations of the rotor core and the permanent magnet. A
modified example shown in FIG. 26 is an example of application to a
structure having a relatively long distance between the outer
circumference of the rotor core and the permanent magnets. In the
rotor of FIG. 26, linear permanent magnets are used. Further, there
are substantially bow-shape magnetic regions 65 to 68 between the
respective permanent magnets and the outer circumference of the
rotor. The periphery of the magnetic regions 65 to 68 form
effective polar opening angles M1 to M4. In this rotor, the
permanent magnets are arranged at an equal pitch, and no deviation
is introduced. However, there is a difference in the configuration
among the magnetic regions 65 to 68. Specifically, in the upper
right and lower left magnetic regions 66 and 68, the right side and
the left side are symmetrical about a pole central line J.
Therefore, the pole central line J and an effective polar opening
angle central line K coincide with each other. However, in the
upper left and lower right magnetic regions 65 and 67, the right
side and the left side are asymmetrical. Hence, there is a
deviation angle .theta.6 between the pole central line J and the
effective polar opening angle central line K. Thus, a reduction in
cogging torque is achieved by the presence or absence of a
deviation angle in each magnetic pole.
FIG. 27 shows an example implemented by replacing the permanent
magnets of FIG. 26 with curved permanent magnets. In this rotor,
there is also the presence or absence of a deviation angle in each
magnetic pole because of the difference in the configuration among
the magnetic regions between the permanent magnets and the outer
circumference of the rotor. Thus, a reduction in cogging torque is
achieved. The modified examples shown in FIG. 26 or FIG. 27 are
applicable to any one of the third through eighth embodiments. It
is not necessarily to limit the permanent magnets to those shown in
FIG. 26 or FIG. 27, and the permanent magnets may be replaced with
permanent magnets of any known configuration, such as V-shape, V-
(concave-) shape with a base, bow- (reversed semicircular-) shape,
or arrangement.
As explained in detail above, in the third through eighth
embodiments, the deviation angle between the pole central line and
the effective polar opening angle central line varies according to
each magnetic pole. Therefore, the individual cogging torques
generated by the respective magnetic poles are not in phase.
Accordingly, a brushless DC motor whose overall cogging torque is
reduced by the cancellation of the individual cogging torques is
realized. In particular, by setting the deviation angle difference
.theta.6 within the range of
0.2.times..theta.7.ltoreq..theta.6.ltoreq..theta.5-(0.2.times..theta.7)
with respect to the slot pitch angle .theta.5 and the slot opening
.theta.7, the cogging torque can be effectively reduced.
Moreover, considering the direction of the deviation angle, if
three types of magnetic poles including a reference magnetic pole,
a magnetic pole having a deviation angle in the leading direction
and a magnetic pole having a deviation angle in the lagging
direction are provided, it is also possible to cancel the cogging
torques by three-phase composition. In this case, even if the
individual cogging torques are asymmetrical waveforms or the like,
it is possible to achieve a particularly significant reduction in
cogging torque. Further, in either of the two-phase and three-phase
cases, by equalizing the number of magnetic poles having each
deviation angle, more satisfactory results are obtained. In
particular, in the three-phase case, by arranging all of the
magnetic poles to have any one of the three deviation angles and
equalizing the number of magnetic poles having each deviation
angle, it is possible to reduce the overall cogging torque to near
zero practically.
Additionally, by arranging adjacent magnetic poles in the
circumferential direction not to have the same deviation angle, the
influence of the deviation angle on the position of the center of
gravity of the rotor is reduced. It is thus possible to minimize
the deterioration of the rotary balance. In particular, in the
three-phase case, by arranging any three adjacent magnetic poles in
the circumferential direction to include all of the three deviation
angles, the rotary balance can be almost perfectly maintained.
Besides, in the case where the rotor is a block construction in
which the rotor is divided in the axial direction, if the blocks
are arranged so that the corresponding magnetic poles in the axial
direction have mutually different deviation angles, there are
merits on both the reduction in cogging torque and the maintenance
of the rotary balance.
Furthermore, the reduction in cogging torque by the difference in
the deviation angle is achievable by providing convex portions
corresponding to the magnetic poles on the outer circumference of
the rotor and introducing a deviation angle in the positions of the
convex portions, instead of introducing a deviation angle in the
effective polar opening angle central lines. In this case, the
deviation in the positions of the convex portions has the advantage
that the influence on the rotary balance is extremely small in
comparison with the deviation in the positions of the permanent
magnets.
Note that in the third embodiment, etc., the magnetic poles having
no deviation between the pole central line and the effective polar
opening angle central line and the magnetic pole having the
deviation angle .theta.6 are taken into consideration, but
"magnetic poles having no deviation" are not essential. In short,
the point is the presence of a relative difference in the deviation
angle between the magnetic poles. Thus, it is also possible to set
a magnetic pole having a deviation angle .theta.0 between the pole
central line and the effective polar opening angle central line as
a standard and provide magnetic poles having a deviation angle
given by the addition of .theta.6 to .theta.0 or the subtraction of
.theta.6 from .theta.0. The same thing can also be said for the
deviation angle of the convex portions in the seventh or eighth
embodiment. Note that a combination of the deviation angle of the
convex portions in the seventh or eighth embodiment and the
divided-block structure is of course available.
(Ninth Embodiment)
FIG. 28 is a perspective view showing the construction of a stator
of a brushless DC motor according to the ninth embodiment of the
present invention. This stator 1 is formed by layering a number of
thin electromagnetic steel plates and fixing them integrally, and
comprises a yoke 2 that is formed as an outer circumferential
portion and teeth 3 that are provided at equal intervals to
protrude from the yoke 2 toward the center. Adjacent teeth 3 form a
slot 4 together with the yoke 2. Actually, armature windings (not
shown) are wound on the teeth 3 and stored in the slots 4.
In the thin electromagnetic steel plate, a portion corresponding to
the notch portion 9a is provided at the outer circumferential
surface portion of the yoke 2 near the outside of a portion
corresponding to every third tooth 3. The portion corresponding to
the notch portion 9a has a protrusion therein so as to facilitate
layering and welding of the thin electromagnetic steel plates.
Blocks are formed by layering substantially an equal number of such
thin electromagnetic steel plates at equal angle so that the notch
portion 9a of every third tooth 3 has substantially an equal length
in the layering direction. These blocks are layered while
displacing them at a predetermined angle in the circumferential
direction so that the portions corresponding to the notch portions
9a are aligned. The notch portions 9a of each block are formed so
that they do not overlap adjacent notch portions 9a in the
circumferential direction.
Accordingly, as shown in the side view of the stator 1 of FIG. 29,
the stator 1 is a construction comprising layers of a multilayer
block segment having a notch portion 9a on the outside of every
third tooth 3 and a thickness S11; a multilayer block segment which
is displaced from the above multilayer block segment by an amount
corresponding to one tooth 3 in the circumferential direction and
has a notch portion 9a on the outside of every third tooth 3 and a
thickness S21; a multilayer block segment which is similarly
displaced from the above multilayer block segment by an amount
corresponding to one tooth 3 in the circumferential direction and
has a notch portion 9a on the outside of every third tooth 3 and a
thickness S31; a multilayer block segment which is similarly
displaced from the above multilayer block segment by an amount
corresponding to one tooth 3 in the circumferential direction and
has a notch portion 9a on the outside of every third tooth 3 and a
thickness S12; a multilayer block segment which is similarly
displaced from the above multilayer block segment by an amount
corresponding to one tooth 3 in the circumferential direction and
has a notch portion 9a on the outside of every third tooth 3 and a
thickness S22; and a multilayer block segment which is similarly
displaced from the above multilayer block segment by an amount
corresponding to one tooth 3 in the circumferential direction and
has a notch portion 9a on the outside of every third tooth 3 and a
thickness S32.
The multilayer block segments with the thickness S11, S21, S31 and
the multilayer block segments with the thickness S12, S22, S32 have
the notch portions 9a at the same positions in the circumferential
direction respectively. Here, the following equation (4) is
satisfied.
Besides, if the total thickness is S0, then the thickness of the
multilayer block segments without the notch portions 9a of each
tooth 3 is given by the following equation (5).
FIG. 30 is a view showing the state of magnetic flux in such a
brushless DC motor. Here, a rotor 5 constructed by attaching
permanent magnets 7 to the surface of a rotor core 6 is disposed in
the stator 1 shown in FIG. 28. The stator 1 has the construction
explained with reference to FIGS. 28 and 29. Note that the rotor 5
may be a buried-type rotor constructed by burying the permanent
magnets 7 in the rotor core 6.
In each portion of the stator 1, magnetic flux generated because of
the relative positional relationship with each region between
magnetic poles of the opposing permanent magnets 7 of the rotor 5
flows. The magnetic flux is shown by indicating the flux amount of
a magnetic path d at a position where the notch portion 9a whose
length in the layering direction is equal to S11 is present on the
outer circumference side of the tooth 3 of the stator 1 as .phi.4,
indicating the flux amount of a magnetic path e which is adjacent
to the magnetic path d and located at a position where the notch
portion 9a whose length in the layering direction is equal to S11
is present on the outer circumference side of the tooth 3 of the
stator 1 a .phi.5, and indicating the flux amount of a magnetic
path f adjacent to the magnetic path e as .phi.6.
Here, as shown in FIG. 30, if straight lines A, B and C are drawn
from the center of the shaft hole of the rotor 5 toward the outer
circumference of the stator 1 through the center of the slots 4,
then, when a region between the magnetic poles of the permanent
magnets 7 of the rotor 5 is positioned on the straight line A, the
magnetic flux from the permanent magnets 7 near the region between
the magnetic poles form a closed circuit of the flux amount .phi.4
by the magnetic path d shown by a dotted line. Moreover, when the
region between the magnetic poles of the permanent magnets 7 of the
rotor 5 reaches the straight line B as a result of clockwise
rotation of the rotor 5, the magnetic flux from the permanent
magnets 7 near the region between the magnetic poles form a closed
circuit of the flux amount .phi.5 by the magnetic path e shown by a
dotted line. When the region between the magnetic poles of the
permanent magnets 7 of the rotor 5 reaches the straight line C as a
result of further clockwise rotation of the rotor 5, the magnetic
flux from the permanent magnets 7 near the region between the
magnetic poles form a closed circuit of the flux amount .phi.6 by
the magnetic path f shown by a dotted line.
For example, suppose that the notch portions 9a are aligned in the
layering direction over a multilayer block thickness S0 of the
stator 1. At this time, the flux amount flowing in the yoke 2
having the notch portions 9a in the layering direction of the teeth
3 is denoted as .phi.1a, and the flux amount flowing in the yoke 2
having no notch portions 9a in the layering direction of the teeth
3 is denoted as .phi.1b. Then, both of portions having the notch
portions 9a and portions having no notch portions 9a in the
layering direction are present in the construction of the stator 1
shown in FIG. 30. Therefore, when the region between the magnetic
poles of the permanent magnets 7 of the rotor 5 is positioned on
the straight line A, the flux amount .phi.4 of the magnetic path d
with the straight line A as the center is given by the following
equation (6).
Similarly, when the region between the magnetic poles of the
permanent magnets 7 of the rotor 5 is positioned on the straight
line B, the flux amount .phi.5 of the magnetic path e with the
straight line B as the center is given by the following equation
(7).
Likewise, when the region between the magnetic poles of the
permanent magnets 7 of the rotor 5 is positioned on the straight
line C, the flux amount .phi.6 of the magnetic path f with the
straight line C as the center is given by the following equation
(8).
Here, it is apparent by substituting equations (4) and (5) for
equations (6), (7) and (8) that the magnetic coupling between the
stator 1 and the rotor 5 can be made stable coupling with less
fluctuation because the flux amounts .phi.4, .phi.5 and .phi.6 of
the magnetic paths d, e and f in the yoke 2 are constant even when
the region between the magnetic poles of the rotor 5 is positioned
on any one of the straight lines A, B and C. In other words, the
cogging torque can never increase locally depending on the
rotational position of the rotor 5, thereby restricting the
generation of sound and vibration resulting from the cogging
torque.
Regarding the magnitude of the cogging toque, as shown in FIG. 31,
since there is no protruding portion on the straight lines A to C,
the torque does not change abruptly and thereby reducing the
cogging torque relatively. In FIG. 31, the vertical axis indicates
the cogging torque TC, while the horizontal axis indicates the
rotation angle .theta. of the rotor 5, and the positions of the
straight lines A to C in FIG. 30 correspond to the positions of the
straight lines A to C in FIG. 31.
It is also apparent from the above description and equations (4) to
(8) that the same effects are obtained even when the notch portions
9a on the outer circumference side of the stator 1 of an arbitrary
tooth 3 are distributed to any positions in the layering direction.
For example, in the construction shown in FIG. 30, if the thin
electromagnetic steel plates are layered by introducing a
displacement corresponding to up to one tooth 3 whenever one thin
electromagnetic steel plate is layered, the notch portions 9a are
aligned whenever three thin electromagnetic steel plates are
layered. Accordingly, if a large number of plates are layered, the
total length, in the layering direction, of the notch portions 9a
provided for each tooth 3 of the stator 1 becomes substantially
equal, and thus the objective of the ninth embodiment can be
achieved.
Besides, for example, in the method of layering the thin
electromagnetic steel plates of the stator 1 by introducing a
displacement of a predetermined angle whenever one thin
electromagnetic steel plate is layered as described above, the
known automatic clamp control for punching and integrally fixing
the thin electromagnetic steel plates at the same time becomes
complicated, and the punching speed can not be increased. However,
by employing a block structure including a plurality of notch
portions 9a aligned in the layering direction, it is possible to
simplify the punching control and increase the punching speed,
thereby improving the productivity.
Further, in the above-described example, for the thin
electromagnetic plates that forms the stator 1, a portion
corresponding to the notch portion 9a is provided at an outer
circumferential portion equivalent to every third tooth 3. However,
as shown in FIG. 32, for the thin electromagnetic plates forming
the stator 1, a portion corresponding to the notch portion 9b may
be provided at an outer circumferential portion corresponding to
every other tooth 3. In this case, even when the notch portion 9b
is provided in the magnetic path, the region between the magnetic
poles of the permanent magnets 7 of the rotor 5 is present to face
any one of the slots 4, thereby achieving the objective of the
ninth embodiment.
If the thin electromagnetic steel plates are layered while
displacing them at a predetermined angle so that the notch portion
9b is provided in the outer circumferential portion of every other
tooth 3, it is possible to punch every notch portion 9b at a
minimum displacement angle corresponding to a single tooth 3, and
thus the number of punching processes is reduced. For example, in
the case where the notch portions 9b are punched by moving the
stator 1 at a desired angle, it is possible to increase the
punching speed and improve the productivity. In other case, if the
total length, in the layering direction, of the notch portions 9b
provided for each tooth 3 of the stator 1 is substantially equal,
the objective of the ninth embodiment can be achieved.
In particular, in the case where a countermeasure against
deterioration of cogging torque is implemented by a slight
adjustment of cogging torque, the notch portions 9a to be provided
on the outer circumference side of the stator 1 are arranged within
the pitch range of the teeth 3 so that they do not overlap each
other in the circumferential direction of the stator 1. In other
words, it is necessary that adjacent notch portions 9a in the
layering direction do not overlap each other.
Here, as shown in FIG. 33, notch portions 9c and 9d are provided on
the outer circumference side of the teeth 3, the pitch angle of the
teeth 3 is made Bp, the central angle corresponding to the outer
circumferential portion of each of the notch portions 9c and 9d is
made Bk, the length of the notch portion 9c on the upper side of
the layer is made S11, the length of the notch portion 9d on the
lower side of the layer is made S21, an outer circumferential
surface where no notch portions 9c and 9d are present is made a
circular-arc portion 40 to obtain the layered condition as shown in
FIGS. 28 and 29.
When Bp<Bk, i.e., when adjacent notch portion 9c and notch
portion 9d in the circumferential direction overlap each other,
since both ends of this overlapped portion protrude, the magnetic
flux easily leaks from the tips of these ends. The amount of this
leakage magnetic flux changes largely depending on a slight
difference in the configuration of the notch portions, and the
magnetic flux intended to pass through the magnetic path of the
yoke 2 of the stator 1 leaks outside. Thus, the flux amount passing
through the yoke 2 in the outer circumferential portion of the
teeth 3 is not stable and varies according to the notch width of
each of the notch portion 9c and notch portion 9d, resulting in
deterioration of cogging torque. In particular, this phenomenon
appears more noticeably as the notch portions 9c, 9d and the
circular-arc portion 40 form a more acute angle. Moreover, a
small-size high-output motor with higher magnetic flux density in
the yoke 2 is largely affected by this phenomenon.
Accordingly, in FIG. 33, the central angle Bk corresponding to the
outer circumferential portion of each of the notch portions 9c and
9d is set so as not to be larger than at least the pitch Bp of the
teeth 3, and the notch portion 9c and the circular-arc portion 40
of the outer circumferential surface are alternately provided on
the outer circumference side of the stator 1. Thus, it is seen from
FIGS. 33, 28 and 29 that the portions corresponding to the notch
portions 9c of the layered thin electromagnetic steel plates are
arranged so as not to overlap the portions corresponding to
adjacent notch portions 9d of the thin electromagnetic steel plates
displaced at an angle in the cross section in the layering
direction.
When the central angle Bk is increased to a maximum, Bp=Bk. In this
case, on the outer circumference of the stator 1 in the side view,
an end of the notch portion 9c and an end of the notch portion 9d
are arranged in contact with each other in the circumferential
direction. Accordingly, there is no protrudent portion where the
tips of the above-mentioned two ends are in contact with each other
and the magnetic flux leaks, and thus the leakage magnetic flux
from the stator 1 to the outside can be significantly reduced.
Note that adjacent notch portions in the layering direction have
been explained with reference to the notch portions 9c and 9d on
the outer circumference of the stator 1, but the same explanation
is applied to all the notch portions shown in FIGS. 28 and 29.
Moreover, by setting Bp=Bk, in the outer circumference of the
stator 1, the portions of the corner sections where burr is created
due to the notch portions 9c and 9d can be reduced to one second,
thereby decreasing fitting defects in fitting into a case or the
like.
Note that while the above-described ninth embodiment illustrates
the notch portions of the outer circumference of the stator, the
cavity portions provided in the outer circumference side of the
stator can be explained in the same manner.
* * * * *